Gallium Nitride (GaN) technology has widely been identified as a preferred high voltage (>600 V) power electronics technology due to its inherent high Johnson limit, which is a relationship between cutoff frequency (fT) and breakdown voltage. Quantitatively, the Johnson limit is a product of fT and breakdown voltage for a particular semiconductor technology such as GaN technology. The Johnson limit for GaN technology is significantly improved over the Johnson limit for silicon technology. As a result, GaN technology is being developed to realize relatively very compact and efficient switching regulators that require small passive filter elements in comparison to silicon technologies. However, challenges remain in utilizing GaN technology for compact and efficient switching regulators as well as other commercial applications. Some of the challenges include achieving low cost, normally off operation, low leakage of drain-to-source current (Ids) and low gate leakage current (Igate), as well as low channel on-resistance (R-on).
Moreover, greater than 600 V power electronic GaN switching transistor devices require low Ids leakage current under a high drain-to-source voltage (Vds) condition. A typical power electronic GaN switch requires less than 10-20 μA/mm of Ids leakage current under 1200 V Vds operation in an off-state in order to minimize the off-power dissipation and maximize switching efficiency. In addition, the same GaN switch requires a very low on-resistance of <200 milli-Ohms in the on-state in order to minimize on-power dissipation and maximize switching efficiency. The on-resistance may be reduced by increasing the overall size of the device by increasing the gate width. However, this will increase cost and the absolute value of Igate leakage current, which is proportional to the gate width.
Excessive leakage current is a common problem with lateral high electron mobility transistor (HEMT) devices. In a lateral HEMT device, a channel surface typically needs to be passivated to reduce surface states that contribute to electron leakage transport in a lateral direction. The leakage current that results from the surface states increases with higher voltage operation and the resulting electric fields. In particular, the leakage currents and breakdown voltages are strongly influenced by peak electric fields between the gate and drain regions of a device. Excessive leakage current is often mitigated with field distribution techniques such as employing sloped gate metal, gate field plates, and source field plates over the gate-drain regions of an active device.